Growing research and development efforts are being made for an ultra high frequency device with an emphasis put on monolithic microwave integrated circuit device which comprises passive elements such as a distributed parameter circuit, a lumped-parameter inductor, a capacitor and a resistor formed on a semi insulating substrate of, for example, gallium arsenide and active elements such as bipolar transistors or field effect transistors each having an active layer formed by using an ion implantation technique, a molecular beam epitaxial technique or a metal organic vapor phase epitaxial growth technique. A typical example of the monolithic microwave integrated circuit device is disclosed in IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, vol. MTT-33, No. 11, November 1985, pages 1231 to 1235. Description is hereinunder made for a three-stage amplifier circuit forming part of the monolithic microwave integrated device with reference to FIGS. 1 and 2 of the drawings.
Referring first to FIG. 1, there is shown the three-stage amplifier circuit accompanied with an input node 1 and an output node 2. The three-stage amplifier circuit comprises micro-strip lines 3 and 4 one of which is coupled at one end thereof to the input node 1 and at the other end thereof to a capacitor 5 and the other of which is coupled at one end thereof to the input node 1 and at the other end thereof to a gate electrode of a field effect transistor 6. The capacitor 5 in turn is coupled at the other electrode thereof to a ground pad 7. The field effect transistor 6 is coupled between the ground pad and a micro-strip line 8 which is coupled in parallel to a capacitor 9 and a series combination of a micro-strip line 10 and a capacitor 11. The capacitor 9 is coupled at the other electrode thereof to a gate electrode of a field effect transistor, and the series combination of the micro-strip line 10 and the capacitor 11 is coupled at the other end thereof to the ground pad 7. Thus, a circuit 13 is constituted by the field effect transistor 6, the micro-strip lines 8 and 10 and the capacitors 9 and 11, and each circuit 14 or 15 is similar in circuit arrangement to the circuit 13, so that component elements of each circuit 14 or 15 are denoted by like reference numerals designating the corresponding component elements of the circuit 13 without description.
The three-stage amplifier circuit shown in FIG. 1 is fabricated on a semi-insulating substrate 16 of gallium arsenide, and the layout thereof is illustrated in FIG. 2. The three-stage FET amplifier is operable at a frequency of the order of 12 GHz. The three-stage amplifier circuit occupies an area measuring about 1.5 milli-meter.times.about 1.7 milli-meter, and the chip is 150 microns in thickness. Each of the micro-strip line is provided with a conductive strip formed of gold and has a width W equal to or greater than about 50 microns. Though not shown in the drawings, the reverse surface of the chip is covered with gold.
However, a problem is encountered in the prior-art microwave integrated circuit device in large occupation areas. This is because of the fact that the micro-strip lines occupy a large amount of area on the substrate in comparison with the active component elements such as field effect transistors. The reasons why the micro-strip lines consume a large amount of area are as follows.
First, it is impossible to reduce each micro-strip line in width to a value less than 50 microns in consideration of the transmission loss of signal. Second, it is necessary for each micro-strip line having a characteristic impedance ranging between 50 ohms and 100 ohms to select the thickness of the semi-insulating substrate 16 of about 150 microns if the semi-insulating substrate 16 is formed of a gallium arsenide with a dielectric constant of the order of 12. In this situation, each micro-strip should be spaced apart from the adjacent micro-strip line by a distance three times greater than the thickness of the semi-insulating substrate 16 for preventing these adjacent micro-strip lines from capacitive coupling. Then, each micro-strip line is arranged to be spaced from the adjacent micro-strip line by at least 450 microns. Finally, if the component element is scaled down in thickness of a dielectric material and in width of the micro-strip line, the characteristic impedance and the propagation constant ( which is assumed to be negligible ) are not affected by the scaling down. This means that each microstrip line is reduced in width but the length of each micro-strip line is unchanged as a result of the scaling down. Thus, the micro-strip lines occupy a large amount of area, and the occupation area is hardly reduced by the prior-art method.